Sealing composition for sealing aluminum nitride and aluminum oxynitride ceramics

ABSTRACT

There is described a sealing composition for sealing aluminum nitride and aluminum oxynitride ceramics comprising: a mixture of SiO 2 , at least one other metal oxide, and a silicon additive comprising at least one of silicon metal or a silicide. The silicon additive acts to suppress the formation of nitrogen bubbles during the sealing of articles comprised of aluminum nitride or aluminum oxynitride ceramics, e.g., as in the case of a ceramic discharge vessel for a high intensity discharge lamp.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 12/327,207,filed Dec. 3, 2008, now U.S. Pat. No. 7,659,220 which is herebyincorporated by reference.

TECHNICAL FIELD

The present invention relates to aluminum nitride and aluminumoxynitride ceramics and in particular to the use of such ceramics forarc discharge vessels in high-intensity discharge (HID) lamps, e.g.,metal halide lamps and high-pressure sodium lamps (HPS).

BACKGROUND OF THE INVENTION

Metal halide and high-pressure sodium lamps are two examples of lampswhich use a discharge vessel made of a ceramic material. The robustnessof the ceramic material permits the use of more corrosive chemical fillsand allows the discharge vessels to be operated at higher temperatures.A sealing composition, commonly referred to as a frit, is used to form ahermetic seal between the feedthrough sections of the electrodes and theceramic body of the discharge vessel. The sealing composition may alsobe used to join together the ceramic pieces which form the dischargevessel.

FIG. 1 shows a discharge vessel 1 for a conventional ceramic metalhalide lamp. The discharge vessel 1 includes a hollow ceramic body 6filled with a chemical fill 8 and into which electrodes 14 are fedthrough ceramic capillaries 2. The discharge vessel has halves 17 a,bthat are joined at seam 5, where the electrode ends 3 are inside therespective halves and extend into the discharge chamber 12. The distalends of the capillaries are each sealed with a respective frit seal 9. Aconventional Al₂O₃—Dy₂O₃—SiO₂ sealing composition for this purpose isdescribed in U.S. Pat. No. 4,076,991.

Translucent polycrystalline alumina (PCA) has been by far the ceramicmaterial of choice for making ceramic discharge vessels. Yet despite itspervasive use, PCA is not used in some HID applications because it isonly translucent and not transparent. In particular, a PCA dischargevessel is generally not suitable for focused-beam, short-arc lamps suchas projection lamps and automotive headlights. For these applications,the transparent sapphire (single-crystal) form of aluminum oxide isused. In addition, PCA discharge vessels although superior to quartz arctubes in metal halide lamps can react with the rare earth halide fillslimiting the durability and life of such lamps.

Aluminum oxynitride (AlON) is a transparent ceramic material that hasbeen identified as a potential replacement for PCA. See, for example,Japanese Patent No. 09-92206 and U.S. Pat. Nos. 5,924,904 and 5,231,062.AlON has a cubic spinel structure and a composition that may begenerally represented by the empirical formulaAl_((64+x)/3)O_(32−x)N_(x) where 2.75≦x≦5. The mechanical strength andthermal expansion of AlON are close to those of PCA, so that AlON shouldbe able to survive the stresses in high-intensity discharge lamps.

Aluminum nitride (AlN) has been shown to be more resistant to thecorrosive effects of rare earth metal halide fills than polycrystallinealuminum oxide. Although the fully-dense sintered AlN ceramic is onlytranslucent and not transparent, the superior corrosion resistance isdesirable for metal halide lamps. For example, the use of AlN arcdischarge vessels for ceramic metal halide lamps is described inEuropean Patent Application No. 0371315A1 and PCT Application No. WO03/060952.

Despite the potential advantages of aluminum nitride and aluminumoxynitride ceramics, there remain a number of technical difficultieswhich must be overcome for these materials to be considered for use inHID lamp applications. One in particular is the reaction of theseceramics with the conventional silica-containing glass/ceramic fritmaterials used to seal the discharge vessels. As described above, thefunction of the frit is to hermetically seal the ceramic body of thedischarge vessel, in particular to the feedthrough portion of theelectrode assembly. During the sealing operation, contact between themolten frit and the aluminum nitride or aluminum oxynitride ceramicinduces a reaction that releases nitrogen gas. Since most of thenitrogen evolved from the reaction cannot be accommodated in the moltenfrit, it escapes as gas bubbles into the frit seal. These gas bubblesmay degrade the quality and function of the hermetic seal leading topremature failure of the lamp, particularly when higher pressures arepresent in the discharge vessel.

U.S. Pat. No. 7,362,053 describes a method for minimizing bubbleformation in the seal region of an aluminum oxynitride discharge vesselby forming a less reactive surface layer in the seal region. In oneembodiment, this is accomplished by heating at least the seal region ofthe discharge vessel in a reducing atmosphere. In another embodiment, analuminum oxide layer is deposited in the seal region to act as a barrierbetween the molten frit and the aluminum oxynitride vessel duringsealing.

While such methods are effective, it would be simpler and moreadvantageous if a sealing frit composition could be formulated in such away to reduce or eliminate bubble formation in the seal without the needfor additional treatments on the sintered discharge vessel.

SUMMARY OF THE INVENTION

It is an object of the invention to obviate the disadvantages of theprior art.

It is another object of the invention to provide a sealing compositionfor sealing aluminum nitride and aluminum oxynitride ceramics thatreduces or eliminates bubble formation in frit seals.

In accordance with an object of the invention, there is provided asealing composition for sealing aluminum nitride and aluminum oxynitrideceramics comprising: a mixture of SiO₂, at least one other metal oxide,and a silicon additive comprising at least one of silicon metal or asilicide. (As used herein, silicon and silica are considered a metal anda metal oxide, respectively.) Preferably, the other metal oxide isselected from Al₂O₃, Dy₂O₃, CeO₂, Gd₂O₃, Y₂O₃, Eu₂O₃, Tm₂O₃, Ho₂O₃, CaO,MgO, BaO, La₂O₃, V₂O₅, WO₃, MoO₃ and ZrO₂. More preferably, the othermetal oxide is selected from Al₂O₃, Dy₂O₃, CeO₂, Y₂O₃, CaO, and MgO. Thesilicide preferably is a silicide of W, Mo, Ta, Nb, Pd, Re, Pt, or Rh.More preferably, the silicide is tungsten silicide or molybdenumsilicide.

In a preferred embodiment, the sealing composition contains at least 1percent by weight SiO₂.

In a more preferred embodiment, the sealing composition consistsessentially of a mixture of silicon metal powder and an oxide mixtureconsisting essentially of 20 wt. % Al₂O₃, 15 wt. % SiO₂ and 65 wt. %Dy₂O₃.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional illustration of a conventional ceramic metalhalide, high intensity discharge lamp.

FIG. 2 is a photomicrograph of a cross section of a frit-sealed aluminumnitride ceramic using a conventional silica-containing frit.

FIG. 3 is a photomicrograph of a cross section of a second frit-sealedaluminum nitride ceramic using the sealing frit composition of thisinvention.

DETAILED DESCRIPTION OF THE INVENTION

For a better understanding of the present invention, together with otherand further objects, advantages and capabilities thereof, reference ismade to the following disclosure and appended claims taken inconjunction with the above-described drawings.

A preferred frit material for sealing ceramic discharge vessels is theDy₂O₃—Al₂O₃—SiO₂ glass-ceramic system. This system is widely used bylighting manufacturers to seal PCA discharge vessels because of itshalide resistance and favorable melting and thermal expansioncharacteristics. The Dy₂O₃—Al₂O₃—SiO₂ frit seal consists of DA(3Dy₂O₃-5Al₂O₃) and DS (Dy—Si—O) crystalline phases in a Dy—Al—Si—Oglassy matrix.

When sealed to PCA parts, some alumina from the PCA part is dissolved inthe frit at the frit-PCA interface, but there are typically no bubblesin the frit seals of the PCA parts. As described previously, this is notthe case when the same frit is used with aluminum nitride or aluminumoxynitride parts.

The general reaction of a divalent metal oxide with aluminum nitride canbe expressed as:2/nMeO_(n)+4/3AlN→2/3Al₂O₃+2/3N₂+2/nMe

For silica (SiO₂) in the sealing frit, the reaction is:SiO₂+4/3AlN→2/3Al₂O₃+2/3N₂+Si

The change in the Gibbs free energy, ΔG, is negative at the temperatureof the sealing process favoring the release of nitrogen during sealing.Analogous reactions occur in the case of aluminum oxynitride alsoresulting in the release of nitrogen gas during sealing. Since not allof the nitrogen can be absorbed into the molten frit material, bubbleformation occurs as shown in the photomicrograph of FIG. 2.

The present invention adds silicon to the silica-containing sealing fritto reduce bubble formation. In particular, a silicon additive in theform of powdered silicon metal or a powdered silicide such as tungstenor molybdenum silicide is used to shift the reaction equilibrium backtowards the reactants thereby decreasing the release of nitrogen andreducing bubble formation. Preferably, the silicon additive comprises0.1 to 5 weight percent (wt. %) of the sealing composition. Morepreferably, the silicon additive comprises 0.5 to 2.5 weight percent(wt. %) of the sealing composition. Other silicides in that may be usedinclude silicides of Ta, Nb, Pd, Re, Pt and Rh.

As used herein, a silica-containing frit means a frit (or sealingcomposition) containing at least 1 wt. % SiO₂. Silica is added for twopurposes: (1) to lower the melting point of the frit and (2) to generatea glassy or amorphous phase. Lowering the melting point is important sothat volatile materials like mercury are not lost during the finalsealing of the discharge vessel. For example, a Dy₂O₃—Al₂O₃ frit meltsat >1760° C. without silica, but adding silica can lower the meltingpoint to ˜1540° C. The addition of silica also gives rise to a glass oramorphous phase instead of the completely crystalline phases observed inSiO₂-free frits. The glassy phases have a gradual and much smallervolume change during solidification so that the seal remains free ofcracks. Crystalline phases typically have an abrupt 5-20% volume changeduring solidification which leads to microcracks in the seal causing theseal to be mechanically weak and lack hermeticity. Examples of othersilica-containing frits include CeO₂—Al₂O₃—SiO₂, Gd₂O₃—Al₂O₃—SiO₂,Y₂O₃—Al₂O₃—SiO₂, and Eu₂O₃—Al₂O₃—SiO₂. In addition to theabove-mentioned metal oxides, the following additional metal oxides mayalso be used in the sealing composition: Tm₂O₃, Ho₂O₃, CaO, MgO, BaO,La₂O₃, V₂O₅, WO₃, MoO₃ and ZrO₂.

EXAMPLES

FIG. 2 is a cross-sectional photomicrograph of an aluminum nitridecapillary that has been sealed with a frit consisting of 20 wt. % Al₂O₃,15 wt. % SiO₂ and 65 wt. % Dy₂O₃ (20Al₂O₃-15SiO₂-65Dy₂O₃). The presenceof gas bubbles in the seal is readily apparent.

FIG. 3 is a cross-sectional photomicrograph of a similar aluminumnitride capillary sealed with the same 20Al₂O₃-15SiO₂-65Dy₂O₃ frit usedin FIG. 1 except that 5 wt. % Si has been added based on the weight ofthe oxide mixture. In this case, there are no discernible gas bubbles inthe frit which indicates that the addition of Si has suppressed therelease of nitrogen during the sealing process.

While there have been shown and described what are at present consideredto be preferred embodiments of the invention, it will be apparent tothose skilled in the art that various changes and modifications can bemade herein without departing from the scope of the invention as definedby the appended claims.

1. A ceramic discharge vessel, comprising: a hollow ceramic bodycomprised of an aluminum nitride or an aluminum oxynitride ceramic, thedischarge vessel being sealed with a sealing composition comprising: amixture of SiO₂, at least one other metal oxide, and a silicon additivecomprising at least one of silicon metal or a silicide.
 2. The sealingcomposition of claim 1 wherein the at least one other metal oxide isselected from Al₂O₃, Dy₂O₃, CeO₂, Gd₂O₃, Y₂O₃, Eu₂O₃, Tm₂O₃, Ho₂O₃, CaO,MgO, BaO, La₂O₃, V₂O₅, WO₃, MoO₃ and ZrO₂.
 3. The sealing composition ofclaim 1 wherein the silicide is a silicide of W, Mo, Ta, Nb, Pd, Re, Pt,or Rh.
 4. The sealing composition of claim 2 wherein the silicide is asilicide of W, Mo, Ta, Nb, Pd, Re, Pt, or Rh.
 5. The sealing compositionof claim 1 wherein the mixture contains at least 1 percent SiO₂ byweight.
 6. The sealing composition of claim 1 wherein the mixturecontains Al₂O₃ and Dy₂O₃.
 7. The sealing composition of claim 1 whereinthe at least one other metal oxide is selected from Al₂O₃, Dy₂O₃, CeO₂,Y₂O₃, CaO, and MgO.
 8. The sealing composition of claim 1 wherein thesilicide is tungsten silicide or molybdenum silicide.
 9. The sealingcomposition of claim 7 wherein the silicide is tungsten silicide ormolybdenum silicide.
 10. The sealing composition of claim 9 wherein themixture contains at least 1 percent SiO₂ by weight.
 11. A ceramicdischarge vessel, comprising a hollow ceramic body comprised of analuminum nitride or an aluminum oxynitride ceramic, the discharge vesselbeing sealed with a sealing composition comprising: a mixture of SiO₂,at least one other metal oxide selected from Al₂O₃, Dy₂O₃, CeO₂, Gd₂O₃,Y₂O₃, Eu₂O₃, Tm₂O₃, Ho₂O₃, CaO, MgO, BaO, La₂O₃, V₂O₅, WO₃, MoO₃ andZrO₂, and a silicon additive comprising at least one of silicon metal ora silicide of W, Mo, Ta, Nb, Pd, Re, Pt, or Rh, the mixture containingat least 1 percent SiO₂ by weight.
 12. The sealing composition of claim11 wherein the silicon additive comprises 0.1 to 5 weight percent of themixture.
 13. The sealing composition of claim 11 wherein the siliconadditive comprises 0.5 to 2.5 weight percent of the mixture.
 14. Thesealing composition of claim 12 wherein the at least one other metaloxide is selected from Al₂O₃, Dy₂O₃, CeO₂, Y₂O₃, CaO, and MgO.
 15. Thesealing composition of claim 12 wherein the silicide is tungstensilicide or molybdenum silicide.